An uninterruptible power supply system has widely been used as a power supply device for supplying AC power to an important load such as a computer system in a stable manner. For example, as disclosed in Japanese Patent Laying-Open No. 2006-109603 (Patent Document 1), an uninterruptible power supply system generally includes a converter for converting AC power to DC power, an inverter for converting DC power to AC power, and a capacitor for smoothing DC voltage input to the inverter. Usually, the converter converts AC power from a commercial AC power supply to DC power, and supplies the DC power to the inverter while charging a power storage device such as a storage battery. The inverter converts DC power to AC power and supplies the AC power to the load. When the commercial AC power supply fails, power from the power storage device is supplied to the inverter, which continues to supply AC power to the load.    Patent Document 1: Japanese Patent Laying-Open No. 2006-109603